Danfoss MCO 305 Operating Instructions Manual Download Page 22

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Influence of the Proportional Factor:
KPROP too small

large position deviation due to non-
compensatable load and frictional
moment

KPROP larger

quicker reaction, smaller steady-state
deviation, larger overshoot, lesser
damping

KPROP too great

heavy vibrations, instability

4.3.1 Derivative Value for PID Control

The

32-61 Derivative factor

is multiplied with the derivative

of the position deviance (the 'velocity' of the position
deviance) and the result is added to the control signal. The
behavior of the derivative control is similar to that of an
absorber – the faster the absorber is extended the stronger
the counter-force it produces. Thus using the Derivative
Value increases damping in your system.

Influence of the Derivative Value:
KDER small

no effect

KDER larger

better dampening, lesser overshoot; if
KPROP is increased simultaneously: faster
reaction to control deviation at the same
level of vibration;

KDER too large

heavy vibrations, instability

4.3.2 Integral Factor: KINT

The sum of all error is calculated every time the control
signal is updated. The

32-62 Integral factor

is then

multiplied with the sum of all positioning errors and added
to the overall control signal.
Thus in case that steady-state position errors occurs in
your application, make sure to use the integral part of the
controller. Steady-state errors will be leveled out as the
summed error increases over time until the control signal
eventually matches the load.
It is possible to set a limit for the control signal generated
by the integral part of the controller (anti-windup).

Influence of the Integral factor:
KINT very small

steady-state position deviance is very
slowly regulated to zero

KINT larger

faster regulation towards zero steady-
state position deviance, larger overshoot

KINT too large

heavy vibrations, instability

4.3.3 Limit Value for Integral Sum: KILIM

The

32-63 Limit Value for Integral Sum

sets a limit for the

control signal generated by the integral part of the
controller. This helps to prevent the so called "wind-up"
problem which typically occurs in applications where the

overall control signal (the internal speed-reference) is
maxed out for long periods of time.
This feature is also very helpful in applications where the
power to the motor is turned off and on while the option
card is controlling the VLT AutomationDrive. Cutting of
power to the motor (by setting terminal 27 low) while little
positioning deviance is present in the controller, could
result in an enormous control signal being generated once
the power is turned back on.

4.3.4 Velocity Feed-forward: FFVEL

The

32-65 Velocity Feed-Forward

is a scaling factor that is

multiplied with the derivative of the setpoint position (the
velocity of the setpoint). The result of this operation is
added to the overall control signal. This feature is
especially useful in applications where there is a good
correlation between the control signal (the VLT
AutomationDrive speed reference), and the speed of the
motor. This is indeed the case with most applications.

NOTE

The scaling of the FFVEL parameter is dependent on the
correct setting of the

Maximum Reference

(

3-03 Maximum

Reference

) as well as the

32-80 Maximum Velocity (Encoder)

and

32-01 Incremental Resolution

4.3.5 Acceleration Feed-forward: FFACC

The

32-66 Acceleration Feed-Forward

is multiplied with the

2nd derivative of the setpoint position (the acceleration of
the setpoint) and the result is added to the control signal.
This feature should be used to compensate for the torque
used to accelerate/decelerate the system inertia.

NOTE

Scaling of the

Acceleration Feed-forward

factor is

depending on the setting of the

Shortest Ramp

. Increase

32-66 Acceleration Feed-Forward

accordingly when

decreasing

32-81 Shortest Ramp

and vice versa.

4.3.6 Sampling Time for PID Control: TIMER

For particularly slow systems, slow down the entire control
system by entering multiples of 1 ms as the sampling time.
However, it is important to remember that such a change
influences all the control parameters!
Therefore, normally you should not deviate from the value
of 1 ms in

32-69 Sampling Time for PID Control

4.3.7 PID Bandwidth

Bandwidth

of 1000 means that the set value is being

executed 100%, thus

Derivative, Proportional

and

Integral

Factors

are effective as defined. But if operating a system

Optimizing the PID controll...

MCO 305 Operating Instructions